you should try with :
<property name="configDir" location="../Server"/>
from the Ant Manual property task :
Sets the property to the absolute filename of the given file. If the value of this attribute is an absolute path, it is left unchanged (with / and \ characters converted to the current platforms conventions). Otherwise it is taken as a path relative to the project's basedir and expanded.
Rule of thumb => when dealing with path use location attribute, otherwise value.